1. Field of the Invention
The invention relates to gravitational field measurements. In one aspect, the invention relates to apparatus and method for measuring the vertical gradient of the vertical gravitational field. In another aspect, the invention relates to such apparatus and method suitable for use in oil and gas exploration, for example, in a borehole.
2. Background of the Invention
The measurement of gravity gradients is of great importance in several technological areas. For example, the mapping of horizontal and vertical gradients finds applications in geophysical investigations, such as prospecting for oil and other minerals.
It is known among geophysicists, for example, that various subsurface structures often indicate mineral deposits, such as oil, gas, and the like, and, further that minute variations in the gravitational field occur in the area of these subsurface structures. In this respect, it is well known that an instrument for continuously measuring the vertical gravity gradient in an oil well environment would permit exploration geophysicists and petrology engineers to differentiate, for example, gas and water bearing strata in the vicinity of a borehole. Accordingly, various instruments have been devised to measure the earth's gravity field and/or components thereof and gradient changes therein with the object of determining the location and extent of such deposits.
One prior art method for determining the vertical gravity gradient for well-logging purposes requires a detailed gravity survey using gravitational field intensity measurements made at two depths typically with a minimum separation, between the two depths on the order of several feet, by a borehole gravimeter, followed by the appropriate calculation to generate the gradient. This method of determining the gravity gradient is time consuming and is inherently incompatible with continuous well-logging practice.
Various other prior art methods and apparatus are also known, such as, for example, those set forth in U.S. Pat. No. 3,630,086 (1971), U.S. Pat. No. 3,668,932 (1972), U.S. Pat. No. 3,926,054 (1975).
Highly desirable are new methods and apparatus for determining gradients directly at the point of reading. Particularly desirable are such methods and apparatus for measuring the vertical gradient of the vertical gravitational field which can be utilized in the vicinity of a borehole for oil and gas exploration, as well as in other applications. Such methods and apparatus for such uses are provided by the invention hereinbelow described.